On Friday, 15 December 2017 10:07:15 UTC+2, wrote:
The mechanical vario is mandatory as per EASA glider certification (as mech. anemometer, mech. altimeter and mech. compass). The electronic vario is usually linked to the computer with many other features (final glide, etc...).
The concept behind that is: if you have a battery failure you still come back home with the 4 essential instruments.

You confuse airspeed indicator (which is mandatory) with variometer.